A. Field of the Invention
The invention relates to a composition suitable for use as a fluid spacer. It also relates to a method of using the spacer composition, particularly in the context of cementing casing in a wellbore.
B. Description of the Prior Art
Rotary drilling techniques are commonly used for drilling wells into the earth. In the rotary drilling of a well, a drilling fluid or mud is circulated between the surface of the earth and the bottom of the well. Drilling muds which are commonly used include water based drilling muds, including both clay and polymer containing muds, oil based drilling muds, and emulsions. Drilling muds serve many useful purposes including the removal of earth cuttings from the well, control of formation pressures, and cooling and lubrication of the drill bit used in forming the well. However, there are also certain detrimental characteristics associated with drilling muds.
Among the problems associated with drilling muds, is that drilling muds tend to flow from the well into exposed permeable formations with the result that mud solids are filtered out on the wall of the well and a filter cake is formed thereon. Even thin filter cakes are detrimental in the completion of wells in that they interfere with obtaining a good cement bond between the wall of the well and the conduit, or casing, positioned in the well. Also, drilling muds frequently contain components which are incompatible with fluids which one may desire to inject into a well containing a mud. For example, it has long been recognized that if certain cement slurries containing free polyvalent metal cations, especially calcium, are brought into contact with muds containing clay or certain polymers, a very viscous and detrimental plug can form in the vicinity of the mud-cement interface. High pressures required to move such a plug can rupture tubing, or make it necessary to stop pumping to avoid rupturing the tubing with the result that appreciable volumes of cement are left inside the tubing. Also, the high pressures can cause fracturing of the formation, thus causing loss of cement to the formation and formation damage. A similar plug forming phenomenon can occur when certain muds are contacted with fluids other than cement, such as certain brines. Another example of mud-cement incompatibility is that lignins, frequently used as dispersants in high density muds, can cause excessive retardation in cements if the cement becomes comingled with the mud.
For these reasons, various techniques have been developed for the removal of drilling muds from a borehole, particularly in the context of injecting a fluid into the borehole where said fluid is not compatible with the mud, and even more particularly, in the context of cementing. A common technique is to employ a "spacer" or "chemical wash". Although it is not always clear in the literature whether a particular fluid is a spacer or a chemical wash, a spacer is generally characterized as a thickened composition which functions primarily as a fluid piston in displacing the mud. Frequently, spacers contain appreciable quantities of weighting materials and also include fluid loss control agents. Chemical washes, on the other hand, are generally thin fluids which are effective principally as a result of turbulence, dilution, and surfactant action on the mud and mud filter cake. Chemical washes may contain some solids to act as an abrasive, but the solids content is generally significantly lower than in spacers because chemical washes are generally too thin to have good solids carrying capacity. Spacers are particularly useful where increased viscosity is required to displace a particular mud, or where high reservoir pressure requires a weighted fluid to maintain the required hydrostatic head.
Oil based chemical washes or spacers are disclosed in Means et al., U.S. Pat. No. 3,086,938; Messenger, U.S. Pat. No. 3,688,845; Hill et al., U.S. Pat. No. 3,749,173; and Motley et al., U.S. Pat. No. 3,820,602 and U.S. Pat. No. 3,849,316. Oil based chemical washes or spacers, however, generally do not perform well with water based muds.
Emulsion spacers or washes are disclosed in Parker, U.S. Pat. No. 3,625,286; Tuttle, U.S. Pat. No. 3,653,441; Carney, U.S. Pat. No. 3,850,248; and in Journal of Petroleum Technology, August, 1974, page 856. Webb, U.S. Pat. No. 3,532,168 teaches a micellar solution. Emulsions, however, do not lend themselves to easy preparation in the field.
Substantially water based fluids have also been proposed for use as spacers or chemical washes. A water buffer fluid is taught in Neft. Khoz., v. 44, n. 12, p. 25-29 (Dec. 1966) (in Russian--English abstract at 1967 Petrol. Abstr. 80611). Williams, U.S. Pat. No. 2,848,051, employed a composition consisting essentially of insoluble inorganic materials in oil or in water. Lawrence, U.S. Pat. No. 2,582,909, formed a spacer gel by reacting a dilute solution of clay based drilling mud with an inorganic metal salt to form an inhibited gel having a viscosity about equal to that of the uninhibited mud itself. Savins et al., U.S. Pat. No. 3,291,211, taught a liquid, which could be either aqueous or organic, containing a dissolved solute to impart viscoelastic properties to the liquid. However, Savins et al. names as suitable solutes for aqueous systems, only organic compounds. In a later patent, U.S. Pat. No. 3,299,952, Savins teaches use of an aqueous solution of polyvinyl alcohol and borate ions. Wieland et al., U.S. Pat. No. 3,878,895, teaches an aqueous mixture of guar gum, a clay stabilizer such as an alkali metal chloride, calcium chloride, or zirconium oxide, a turbulence inducer such as the condensation product of mononaphthalene sulfonic acid and formaldehyde, an acid-soluble particulate such as powdered limestone, and polyethyleneimine. Maly et al., U.S. Pat. No. 3,866,683, teaches using as a spacer, a gelled aqueous hydrated clay suspension. An aqueous spacer which has been employed commercially comprised an aqueous gel of bentonite and guar gum. Another commercial aqueous based spacer is discussed in Society of Petroleum Engineers Preprint 1772 (1967). McKinney et al., U.S. Pat. No. 3,411,580, employed an aqueous gel comprised of a polysaccharide and a crosslinking agent therefor, such as boric acid. While some of these systems have found some measure of commercial success, most have suffered from at least one deficiency, such as compatibility with only a rather limited number of muds, a tendency to flocculate when used with certain cements, difficulty in mixing, susceptibility to bacterial degradation, difficulty in cleaning of equipment, and waste disposal problems. Accordingly, the industry has continued to search for new and improved spacer and wash compositions.
Other relevant art includes Messenger, U.S. Pat. No. 3,467,193, which taught use of an aqueous preflush containing a dispersant, which was followed by a spearhead of cement slurry containing a turbulence inducer, which was followed in turn by a densified cement slurry. Messenger, U.S. Pat. No. 3,884,302, used an aqueous alkaline flush having a pH of at least 10, which was followed by an aqueous fluid which formed a viscous film at the interface with the alkaline flush. Messenger, U.S. Pat. No. 3,348,612, further illustrates the state of the art. Crinkelmeyer, U.S. Pat. No. 3,605,897, teaches that glycols can be used to separate slugs of fluids. A surfactant is taught in J. Jap. Ass. Petrol. Technol. v. 35, No. 2, pp. 83-88, March 1970 (in Japanese; English abstract at Petrol. Abstr. v. 11, part 2, entry 143,459 (Apr. 17, 1971)). Various surfactant systems are reviewed at Neft. Khoz. No. 12, pp. 9-12, December, 1974 (in Russian; English abstract at Petrol. Abstr. v. 15, part 2, entry 204,636 (1975)).
None of the foregoing art suggests to employ as a spacer, water gelled with a water soluble silicate and a water soluble metal compound containing a multivalent metal cation capable of reacting with the silicate.
It has been known to use a water soluble silicate and polyvalent metal salt in cements, grouts, and soil sealing compositions, but not as a spacer. Peeler, Jr., et al., U.S. Pat. No. 3,335,018, taught use of a composition comprising water, a silicate, an amide, neat cement, and a salt reactive with the silicate as a soil sealant. Sullivan, U.S. Pat. No. 2,274,566, taught a grouting composition comprised of paper, water, sodium silicate, and a reactive salt. Japanese Pat. No. 45-7819; Scripture, Jr., U.S. Pat. No. 1,982,541; Drummond, U.S. Pat. No. 2,336,723; and Australian Pat. No. 292,794 also contain similar teachings. Canada Pat. No. 733033 disclosed a slurry comprised of whipped or beaten reaction products of a water solution of sodium silicate and flocculating or gelling agents for such silicates, optionally including sand, cements, or weighting materials. The slurry is said to be useful as a grout or drilling mud.
The teachings of each of the patents and other publications hereinabove specifically referred to are expressly incorporated herein.